What week does the second trimester start?

Week 14. The first trimester runs weeks 1–13, the second runs weeks 14–27, and the third runs from week 28 until delivery.

Why aren't trimesters just 40 weeks divided by 3?

A strict division would give roughly 13.3-week blocks, but the medically used boundaries — weeks 13 and 27 — are set by clinical milestones (end of major organ formation, viability threshold) rather than by splitting 40 evenly. The third trimester ends up the longest at about 12-13 weeks depending on delivery timing.

Is gestational age the same as fetal age?

No. Gestational age is counted from the first day of the last menstrual period, about two weeks before conception actually happens. Fetal (conceptional) age runs about 2 weeks behind gestational age. Due-date calculators and prenatal care both use gestational age by convention.

What happens after week 40?

A pregnancy that continues past 40 weeks (280 days from LMP) isn't automatically abnormal — ACOG defines full term as 39w0d–40w6d, and pregnancies are generally monitored more closely, with induction commonly discussed, once they pass 41 weeks.

Guide

What trimester am I in? Week ranges and how it's counted

Trimesters split gestation into three unequal chunks — not three neat 13-week thirds.

By Buğra SözeriPublished September 25, 2026

“What trimester am I in” sounds like simple division — 40 weeks split three ways should give about 13.3 weeks each. That’s not how it works. The three trimester boundaries are set by clinical milestones, not arithmetic, and they land at weeks 13 and 27 rather than at even thirds.

The three trimester ranges

Trimester	Gestational weeks	Roughly
First	Weeks 1–13	Months 1–3
Second	Weeks 14–27	Months 4–6
Third	Week 28 to delivery	Months 7–9(+)

Gestational age itself is counted in weeks and days from the first day of the last menstrual period (LMP) — not from conception, which typically happens about two weeks later. That's why a pregnancy is described as “40 weeks” even though the embryo has only existed for closer to 38 weeks by the time of a typical due date; the extra two weeks are counted before conception even occurs, by convention.

Why the boundaries fall where they do

The first trimester ends around week 13 because that’s roughly when the period of highest miscarriage risk and major organ formation tapers off. The second trimester ends around week 27 because week 28 is close to the threshold where a fetus historically became considered viable outside the womb with intensive care — a marker used clinically even though outcomes have improved at earlier gestational ages since. Neither boundary is arbitrary, but neither is a clean division of 40 by 3 either.

This unevenness is also why the third trimester feels longest in practice — at roughly 12–13 weeks (28 through delivery), it runs about the same length as the first two combined minus a week, not a neat third of the total.

Trimester from your due date, worked example

If your LMP was January 1 and today is week 30 of gestation, you’re in the third trimester (weeks 28+). If you were at week 20, you’d be squarely in the second trimester. The due date calculator does this lookup automatically — it computes your current gestational age from either your LMP or conception date and maps it straight to a trimester, so you don’t have to count weeks by hand each time you check.

A pregnancy carried past 40 weeks doesn’t get a “fourth trimester” label medically — it’s described instead using term categories (early-term, full-term, late-term, post-term), which is a separate classification from the three-trimester system. See full-term vs. post-term pregnancy for how those labels are defined.

Frequently asked questions

What week does the second trimester start?: Week 14. The first trimester runs weeks 1–13, the second runs weeks 14–27, and the third runs from week 28 until delivery.
Why aren't trimesters just 40 weeks divided by 3?: A strict division would give roughly 13.3-week blocks, but the medically used boundaries — weeks 13 and 27 — are set by clinical milestones (end of major organ formation, viability threshold) rather than by splitting 40 evenly. The third trimester ends up the longest at about 12-13 weeks depending on delivery timing.
Is gestational age the same as fetal age?: No. Gestational age is counted from the first day of the last menstrual period, about two weeks before conception actually happens. Fetal (conceptional) age runs about 2 weeks behind gestational age. Due-date calculators and prenatal care both use gestational age by convention.
What happens after week 40?: A pregnancy that continues past 40 weeks (280 days from LMP) isn't automatically abnormal — ACOG defines full term as 39w0d–40w6d, and pregnancies are generally monitored more closely, with induction commonly discussed, once they pass 41 weeks.

Sources & references

Authoritative references cited by this piece. Verified by Buğra Sözeri on the dates shown and re-checked at every deploy.

ACOG — Prenatal Care: Trimesters and Expected Visits — ACOG's patient-facing breakdown of the three trimesters and what each covers(as of 2026-09-25)
CDC — Reproductive Health: During Pregnancy — CDC reference on the stages and general timeline of pregnancy(as of 2026-09-25)